When an endless conveyer belt is repeatedly bent at regions of the head pulley and the tail pulley, the rubber cover of the belt may curl up at the end portion, and consequently detach phenomenon at the endless joint if adhesion is not sufficient. The endless conveyer belt is particularly susceptible to such detachment in a case where the upper surface rubber cover is thick. Once the detachment of the end portion of the rubber cover reaches the core canvas cloth, the conveyer belt may break due to abrasion and deterioration.
Accordingly, used are the methods of joining a layered belt body, comprising rubber covers and a core canvas cloth sandwiched therebetween, at the end portions thereof to form a conveyer belt in an endless configuration, wherein the upper surface rubber cover of each end portion of the belt body is cut from the core canvas cloth towards the outside in a step-like pattern to form a joining surface, and the joining is performed while each end portion of the belt body pressed against each other (Patent Documents 1 and 2).
Meanwhile, in order to achieve the adhesion of a canvas cloth to a rubber cover which comprises a conveyer belt, a coating rubber-forming material is used between the rubber cover and the canvas cloth, or between canvas cloths. Alternatively, the coating rubber-forming material is used for both between the rubber cover and the canvas cloth and between canvas cloths. While good adhesiveness between the coating rubber and the canvas cloth results in excellent durability of the conveyer belt, the peeling work intended for exposing the core canvas cloth during the endlessing of the conveyer belt often tends to become difficult. This also applies to between canvas cloths. To facilitate the endlessing workability, an approach comprising increasing the amount of calcium carbonate contained in the coating rubber-forming material is used. This approach, however, may disadvantageously result in deteriorated durability.
The present applicant has previously proposed a coating rubber composition for conveyer belts as a coating rubber-forming material used for adhesion between a rubber cover and a canvas cloth and/or between canvas cloths when producing an endless belt by joining the end portions of a conveyer belt which is configured so that a canvas cloth is adhered to a rubber cover, wherein the coating rubber composition has an appropriately attenuated adhesive power to allow for facilitation of the endlessing workability, while improving the adhesion durability between the rubber cover and the canvas cloth or between canvas cloths as compared with a case where the peel force is reduced due to increased amount of calcium carbonate in the coating rubber-forming material. That is, proposed is a coating rubber composition for conveyer belts in which carbon black and calcium carbonate are compound as fillers in diene-based rubber, comprising, blended rubber consisting of 82.5 to 52.5 mass % of diene-based rubber and 17.5 to 47.5 mass % of rubber component from reclaimed rubber, and 10 to 85 parts by mass of calcium carbonate per 100 parts by mass of the blended rubber (Patent Document 3).
In a case where the above coating rubber composition for conveyer belts is used as a coating rubber-forming material intended for between a rubber cover and a canvas cloth of a conveyer belt, between canvas cloths or both of these in order to form an reinforced layer, the adhesive power is appropriately attenuated to facilitate the endlessing workability involving peeling, while maintaining sufficient durable adhesiveness between them. Therefore, the above coating rubber composition can sufficiently achieve the desired objective of allowing the conveyer belt to operate stably over a prolonged period.
Here, in view of durability, the coating rubber used between the rubber cover and the canvas cloth or between canvas cloths is required to have good adhesiveness in addition to a high peel force. Generally, when a high peel force is required, carbon black having a small particle diameter is compounded with the coating rubber to obtain the high peel force. In this case, however, the rubber attachability (when visually observing rubber adhering onto the surface of a canvas cloth after peeling tests, cases where disruptions due to the peeling are occurring on the whole surface of the coating rubber layer are taken as 100%) on the peeled surface will decrease. Meanwhile, a method consisting of compounding carbon black having a large particle diameter and a large amount of calcium carbonate may be used to improve the rubber attachability on the peeled surface. In this case, however, the peel force will decrease. As described above, the peel force and the rubber attachability on the adhesion surface are in a contrary relationship, and thus cannot be satisfied simultaneously.
Furthermore, the present applicant has proposed a coating rubber composition for conveyer belts as a coating rubber-forming material used for adhesion between a rubber cover and a canvas cloth and/or between canvas cloths, the coating rubber composition capable of yielding a vulcanized material simultaneously having both improved peel force and rubber attachability between the rubber cover and the canvas cloth or between canvas cloths, and comprising carbon black and calcium carbonate compounded as fillers in diene-based rubber, wherein the coating rubber composition comprises blended rubber consisting of 83.0 to 97.5 mass % of the diene-based rubber and 2.5 to 17.0 mass % of rubber component from reclaimed rubber, and 45 to 75 parts by mass of carbon black having a nitrogen adsorption specific surface area of 30 to 50 m2/g and a DBP oil absorption of 95 to 125 ml/100 g, and 5 to 50 parts by mass of calcium carbonate per 100 parts by mass of the blended rubber (Patent Document 4).
Furthermore, while adhesiveness with the canvas cloth has been the major issue for conventional coating rubbers, there is a need to reduce energy loss at the core portion to confer a power-saving capability on a conveyer belt. There have been demands for a coating rubber capable of reducing energy loss which occurs when a moving conveyer belt bends due to objects loaded onto it.
Moreover, when used as an endless belt in which the ends of the belt body are joined to each other, the belt is exposed to over-vulcanization state due to a prolonged vulcanization such as re-vulcanization at the endless portion. Therefore, adhesion performance also needs to be sufficiently achieved even when over-vulcanized.
However, the rubber attachability value with the canvas cloth is low when over-vulcanized, and good adhesiveness is difficult to obtain. In order to improve the rubber attachability upon over-vulcanization, an adhesive-based compounding agent can also be compounded. This, however, cannot prevent the peel force from reducing when normally vulcanized.